Tuesday, March 12, 2013

Spring Runoff at Risk

It's not time to panic yet, but I've been looking at the forecast for the next couple of weeks and it is bad news for the spring runoff. Much can happen in late March and April, but it appears we will need to make up a lot of ground if we are to have anything close to a decent runoff this spring in the Great Salt Lake Basin.

First, let's have a look at the current snowpack situation. The Bear, Weber, Ogden, and Provo/Jordan river basins are all sitting at about 75% of average snowpack.

Source: NRCS

This week, we have a high-amplitude ridge building in. The 700-mb temperatures are forecast to climb to about +5ºC over Salt Lake City by Thursday afternoon.

Thursday will easily be the warmest day of the year. Portions of the Salt Lake Valley will see maximum temperatures in the low 70s. I think 70ºF will be broken at the Salt Lake Airport if the lake breeze is kept at bay.

All of this means that we will be chipping away at the mountain snowpack during a month that is critical for building the snowpack for the spring runoff. Ideally, you want the mountain snowpack to build into April, followed by a rapid melt and runoff pulse over about a six week period during the late spring. This maximizes the fraction of snow that is converted to runoff. A slow melt of a meager snowpack over an extended period results in a weaker pulse. Not only is the water stored in the snowpack lower, but the fraction of snow converted to runoff is lower because you lose more of it to evaporation, sublimation (ice converting to water vapor), and transpiration by plants (once they become active).

The models suggest a gradual cooling trend after Thursday, but we're only brushed by some weaker systems through the middle of next week. Thereafter, the forecast suggests the dice are loaded for below average precipitation through March 25th.

Source: CPC

As indicated on the Snowbird SNOTEL plot below, the trajectory is not good. Chances are pretty good that we will be behind last year's snowpack if the forecast holds.

Source: NWS/CBRFC

As I mentioned, a lot can happen in the spring, but the odds we can recover dwindle with each passing day and week. Enjoy the spring conditions now, but hope for a return of winter later.

What can I say. The climate has a lot of variability built into it. Just remember that a bad year in the Cottonwoods is better than a good year most anywhere else. Alta has recorded 340 inches of snow this year, which is pretty close to an average year at mid- and upper-elevation observing sites at most "snowy" Colorado resorts (e.g., Vail, Winter Park, Steamboat) and more than what they see at many others. If you think this is bad, move there...

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The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

Participants include aspiring and old-school atmospheric scientists, weather enthusiasts, powder snobs, and poor souls enrolled in classes taught by University of Utah Atmospheric Sciences Professor Jim Steenburgh. Many posts feature content or insights enabled by the support of the National Science Foundation, Office of Naval Research, and the NOAA/National Weather Service.

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